The present invention relates to an antenna tuning controller and, more particularly, to an antenna tuning controller for tuning an antenna by taking impedance matching between the antenna and a radio section.
A prior art radio equipment has an antenna tuner which optimizes a reception sensitivity at a reception frequency. The antenna tuner is provided between the antenna and a radio section for amplifying and frequency converting the received signals. The antenna tuner includes a variable capacitor, the capacitance of which is adjusted in the manufacture of the radio equipment.
However, whenever the reception frequency is changed or it is necessary to cope with a plurality of reception frequencies, the capacitance adjustment should be made to optimize the reception sensitivity. Japanese Laid-Open Patent Publication No. 4-150421 discloses a radio selective call receiver, in which the capacitance of a variable capacitor in an antenna tuner is varied according to a signal representing the level or amplitude of the received signal. In this receiver, a change in the capacitance of the antenna tuner variable capacitor causes a change in its internal impedance, according to which the antenna side impedance and the radio section side impedance are matched, thus causing the radio section output signal level to change. Accordingly, in this receiver the antenna tuner is controlled so as to always provide a high radio section output signal level, i.e., provide for impedance matching between the antenna side impedance and the radio section side impedance, at the selected reception frequency.
However, in the disclosed receiver the antenna tuner is controlled according to the received signal level. This means that when a mixed modulation signal is produced by signals which are not to be received, the antenna tuner impedance is nevertheless controlled according to the level of the mixed modulation. For example, near the base station the electric field intensity of the mixed modulation of signals other than the signal to be received is increased. If the electric field intensity of the mixed modulation of signals exceeds a predetermined value, the impedance of the antenna tuner is controlled so as to increase the level of the mixed modulation signal even though the receiver is not receiving a signal designated for that receiver.
When the user carrying the receiver is moved away from the base station neighborhood and the receiver receives the signal to be received in place of the mixed modulation signal, the antenna side impedance is suddenly changed due to a great frequency difference between the two signals. At the time of the sudden change in the antenna side impedance, the radio section side impedance is not changed, thus giving rise to erroneous impedance matching between the two impedances. Although the antenna tuner impedance is controlled according to the level of the received signal, a certain time is taken until the impedance matching between the antenna and the radio section is obtained afresh because of the sudden change in the antenna side impedance. A time is thus required until reaching of an increased level of the signal to be received. During this time, the received signal may fail to be decoded accurately. Particularly, the receiver fails to receive the own call number that may be transmitted from the base station during this time. In this case, the receiver can not inform the call, and its carrier can not recognize the call.